Reliable piezo-resistive pressure sensor

ABSTRACT

A pressure sensor system for measuring the pressure of a corrosive media includes a silicon plate forming a diaphragm and a glass plate or ring bonded to said silicon plate with an opening over the diaphragm. The diaphragm has resistive areas of different orientations to provide first resistive areas which have increased resistance with diaphragm deflection, and other areas which have decreased or little change in resistance with diaphragm deflection. The resistive areas may be formed by doping the silicon plate. The resistive areas have broad doped connectors extending outward to areas beyond the seal between the glass plate or ring, to wire bond areas on the silicon plate. Accordingly, the wire bond pads are not exposed to the corrosive media.

FIELD OF THE INVENTION

This invention relates to pressure sensors which are subject to adverseor corrosive media.

BACKGROUND OF THE INVENTION

Piezo-resistive pressure sensors are known, and may include a silicondiaphragm with conductive or resistive areas on the surface of thediaphragm. The resistive areas are arranged so that, as the diaphragmdeflects, some of the resistive areas increase in resistance, whileother areas decrease in resistance or are relatively unchanged. Usingappropriate output circuitry, such as a Wheatstone bridge, the outputchanges are generally proportional to the applied pressure and theresulting deflection of the diaphragm.

The resistance areas and immediately associated connections may beimplemented by diffusion of n-type material or p-type material into thesurface of the silicon diaphragm. One pair of resistors may have theresistive areas extending radially, while another pair of resistors mayhave the resistive areas extend circumferentially or perpendicular to aradial line from the center of the diaphragm. Coupling to externalcircuitry is normally accomplished by wire bonding to pad areas on thesilicon diaphragm.

However, in some cases, failure or degradation of the operation of thepressure transducers as described above, have occurred.

SUMMARY OF THE INVENTION

It has been determined that lack of reliability in some cases has been aresult of the corrosive media being measured, attacking the wire bondingconnections. This problem may particularly occur when the corrosivemedia involves diesel engine exhaust fumes, or other similarly corrosivemedia.

The present invention overcomes the problem outlined above by providinga silicon-glass construction in which an apertured glass plate or ringis bonded to a silicon plate into which the diaphragm has been formed.The silicon plate has areas which are not covered by the glass ring orplate, and the resistive areas of the diaphragm have associatedsemi-conductive lead-in areas which extend beyond the glass ring orplate. The port for the media under test is coupled directly to theopening in the glass plate. Wire bond areas on the silicon plate areprovided in locations where the glass plate or ring does not cover thesilicon plate. Accordingly, the wire bond pads and wires are protectedfrom the corrosive media.

Additional features of the construction may include the following:

1. The area of the diaphragm exposed to the corrosive media ispassivated by a very thin layer of glass or silicon nitride.

2. Fusion or adhesive bonding of the silicon plate to a substrate, whichmay also be formed of semi-conductive material, such as silicon.

3. The use of an adhesive between the silicon plate and the glass ringor plate, with the adhesive being impervious to the corrosive medium.

4. Instead of silicon, the diaphragm may be formed of othersemiconductive or other materials.

Other objects, features and advantages of the invention will becomeapparent from a consideration of the drawings and the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a pressure transducer illustratingthe principles of the invention;

FIG. 2 is a top plan view of the pressure transducer of FIG. 1, with thepressure port not being shown;

FIG. 3 is a more detailed view of the silicon plate showing oneillustrative configuration of the resistive elements; and

FIG. 4 shows a Wheatstone bridge output circuit.

DETAILED DESCRIPTION OF THE INVENTION

While the specification describes particular embodiments of the presentinvention, those of ordinary skill can devise variations of the presentinvention without departing from the inventive concept.

Referring more particularly to the drawings, FIG. 1 is a cross sectionalview of a pressure transducer including a silicon plate 12 and anapertured glass plate 14 bonded to the silicon plate 12. The siliconplate 12 is thinned down, preferably by etching, to provide a diaphragm16. The silicon plate 12 is bonded to a substrate 18; and a highpressure port 20 is secured to the glass plate 14 by adhesive 22.

The upper surface of the silicon plate 12 may be passivated by a thenlayer 24 of glass or silicon nitride. Output from the transducer isaccomplished by wire bond leads 26, which are conductively connected toareas on the upper surface of silicon plate 12, in an area where theglass ring or plate 14 does not cover the silicon plate 12.

The substrate 18 has an opening 28 aligned with the diaphragm 16 and theopening 30 in the glass plate 28. Incidentally, the deflection ofdiaphragm 16 will be a function of the pressure on both sides of it, andit therefore constitutes a differential pressure transducer.

Turning now to FIG. 2 of the drawings, it shows the glass plate 14overlying the silicon plate 12, with the exposed areas 34 of the siliconplate 12 having the wires 26 coupled to the surface conductive pathsfrom the resistive areas on the diaphragm. As indicated at referencenumeral 40 in FIG. 2, the diaphragm and/or the openings 30 and 28 in theglass plate 14 and/or substrate 18, may be either round or square, or ofother convenient shapes.

FIG. 3 is an alternative diagrammatic showing of a pressure transducerwith the resistive areas on the diaphragm 16′ being shown. In thisregard, note the resistive areas 52 and 54 mounted radially on thediaphragm 16′. The active resistive elements 52 and 54 areinter-connected by the broader pads 56 and 58. As the diaphragm flexes,the length of the resistive elements 52 and 54 increases, and the widthdecreases, this causing an increase in resistance. This is in contrastto the resistive elements 60 and 62 which extend circumferentially orperpendicular to a radial line, so that their width increases or islittle changed, and the resistance decreases to some extent or isunchanged.

In FIG. 3 the dashed lines 64 or 66 represents the location of the glassplate opening which is generally coextensive with the diaphragm; and oneconfiguration of the glass pate is also defined by dashed lines in FIG.3. At the left hand side of FIG. 3, the wires 68 are coupled to thesemi-conductor leads or printed circuit leads 70 at the wire bond padarea 72.

FIG. 4 is a simple Wheatstone bridge circuit with resistors 52′, 54′,60′ and 62′ connected as indicated. The fixed voltage input source isindicated at reference numeral 74 and the output appears at leads 76.

When the diaphragm is not deflected, the bridge is balanced, and thesame voltage appears at the two output leads. However, when thediaphragm deflects, resistors 52′ and 54′ increase in resistance, andresistors 60′ and 62′ decrease, or are not changed significantly. Theresult is unbalancing of the bridge to provide a difference in potentialacross the output leads 76. An appropriate differential voltage detectorand amplifier 78 is connected to output terminals 76.

Incidentally, basic pressure transducers such as that shown in FIG. 3,but terminating at the conductive areas just outside the diaphragm, areavailable commercially, from either of the two following companies: GENovaSensor, 1055 Mission Court, Fremont, Calif. 94539 and SensoNor, ASA,P.O. Box 196, N-3191 Horten. N, Norway.

With regard to the formation of the diaphragm and bonding to thesubstrate, reference is made to U.S. Pat. No. 5,578,843 granted Nov. 26,1996, and that patent is hereby incorporated by reference into thisspecification. Reference is also made to U.S. Pat. No. 5,349,867 whichshows a pressure transducer including Wheatstone bridge and differentialvoltage detection and amplification circuitry.

In closing, in the foregoing specification and in the drawings,preferred embodiments of the invention are disclosed. However, it is tobe understood that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention. Thus, by way ofexample and not of limitation, other semi-conductors or other materialsmay be employed as the diaphragm, and also for the substrate and for theglass plate. The diaphragm and the associated openings in the substrate,the glass plate or ring and the pressure input port may be circular,square, or of other shapes. The glass plate or ring may be a ringclosely limited to the diaphragm or may be an apertured plate extendingover the silicon plate with only a small exposed area. Instead ofresistance sensing, capacitive sensing may be employed. Accordingly theinvention is not limited to the exact configuration and construction asdescribed hereinabove.

1. A pressure transducer system comprising: a silicon plate having areduced thickness area forming a diaphragm; a glass plate or ringoverlying and bonded to the silicon plate and having an opening over thediaphragm, the silicon plate having an exposed extent beyond the glassplate or ring; resistive areas formed on the surface of the diaphragm,wherein radially extending resistive areas formed on the diaphragmincrease resistance with deflection of the diaphragm, and resistiveareas formed on the diaphragm perpendicular to the radially extendingresistive areas reduce resistance or have little change in resistancewith deflection of the diaphragm; conductive paths coupling wire bondpads on the exposed extent of the silicon plate and the resistive areas;and circuitry coupled to the wire bond pads for providing an outputsignal that varies with the pressure applied to the diaphragm.
 2. Apressure transducer system as defined in claim 1, further comprising avery thin passivation layer extending over the surface of the diaphragm.3. A pressure transducer system as defined in claim 1, furthercomprising an input pressure port coupled to the glass plate and beingbonded thereto by adhesive that is impervious to the media being sensedby the system.
 4. A pressure transducer system as defined in claim 1wherein the circuitry includes a Wheatstone bridge.
 5. A pressuretransducer system as defined in claim 1, wherein the glass plate and thesilicon plate are anodically bonded to one another.
 6. A pressuretransducer system as defined in claim 1 wherein the surface of thesilicon plate facing the glass plate or ring is substantially flat andthe other side of the silicon plate has an etched recess forming thediaphragm.
 7. A pressure transducer system comprising: a semiconductorplate having a reduced thickness area forming a diaphragm; a glass plateor ring overlying and bonded to the semiconductor plate and having anopening over the diaphragm, the semiconductor plate having an exposedextent beyond the glass plate or ring; resistive areas formed on thesurface of the diaphragm, wherein radially extending resistive areasformed on the diaphragm to increase resistance with deflection of thediaphragm, and resistive areas formed on the diaphragm perpendicular tothe radially extending resistive areas reduce resistance or have littlechange in resistance with deflection of the diaphragm; conductive pathscoupling wire bond pads on the exposed extent of the semiconductor plateand the resistive areas; and circuitry coupled to the wire bond pads forproviding an output signal that varies with the pressure applied to thediaphragm.
 8. A pressure transducer system as defined in claim 7,further comprising a very thin passivation layer extending over thesurface of the diaphragm.
 9. A pressure transducer system as defined inclaim 7, further comprising an input pressure port coupled to the glassplate and being bonded thereto by adhesive that is impervious to themedia being sensed by the system.
 10. A pressure transducer system asdefined in claim 7, wherein the circuitry includes a Wheatstone bridge.11. A pressure transducer system as defined in claim 7, wherein theglass plate and the semiconductor plate are anodically bonded to oneanother.
 12. A pressure transducer system as defined in claim 7, whereinthe surface of the semiconductor plate facing the glass plate or ring issubstantially flat and the other side of the silicon plate has an etchedrecess forming the diaphragm.
 13. A pressure transducer systemcomprising: a semiconductor plate having a reduced thickness areaforming a diaphragm; a glass plate or ring overlying and bonded to thesemiconductor plate and having an opening over the diaphragm, thesemiconductor plate having an exposed extent beyond the glass plate orring; sensors coupled to the diaphragm and that vary in electricalproperties with deflection of the diaphragm, wherein at least two of thesensors are perpendicular to each other; conductive paths coupling wirebond pads on the exposed extent of the semiconductor plate and thesensors; and circuitry coupled to the wire bond pads for providing anoutput signal that varies with the pressure applied to the diaphragm.14. A pressure transducer system as defined in claim 13, furthercomprising a very thin passivation layer extending over the surface ofthe diaphragm.
 15. A pressure transducer system as defined in claim 13,further comprising an input pressure port coupled to the glass plate andbeing bonded thereto by adhesive that is impervious to the media beingsensed by the system.
 16. A pressure transducer system as defined inclaim 13, wherein the circuitry includes a Wheatstone bridge.
 17. Apressure transducer system as defined in claim 13, wherein the glassplate and the semiconductor plate are anodically bonded to one another.18. A pressure transducer system as defined in claim 13, wherein thesurface of the semiconductor plate facing the glass plate or ring issubstantially flat and the other side of the silicon plate has an etchedrecess forming the diaphragm.